Optical inspection method and apparatus utilizing a variable angle design

ABSTRACT

Method and apparatus for optical inspection of an article are presented. The apparatus comprises an illumination unit and at least one detection unit. The illumination unit generates an incident beam and directs it onto a predetermined region of the article. The detection unit includes a light collection system and a detector. The light collection system collects light scattered from the illuminated region with a predetermined constant maximum collection angle, and utilizes a variable angle design for selectively selecting from collected light at least one light component propagating with a solid angle segment of the maximum collection angle, and directing it to the detector.

FIELD OF THE INVENTION

[0001] The present invention is in the field of automatic opticalinspection techniques and relates to a method and an apparatus forinspecting articles utilizing a variable angle design.

BACKGROUND OF THE INVENTION

[0002] The manufacture of various articles, such as integrated circuits,printed circuit boards, photolithographic masks, etc., requires them tobe automatically inspected during progress on a production line. Thetimely detection of anomalies on the surface of such article is a veryimportant factor subsequently leading to an increase in productionyields.

[0003] Semiconductor wafers are inspected prior to and after patterningprocedure. Optical inspection systems typically employ such mainconstructional parts as illumination optics and collection-detectionoptics for, respectively, directing incident light from a light sourceonto the wafer to be inspected, and collecting light returned(scattered, reflected) from the wafer and directing the collected lightonto a sensing means.

[0004] Prior-to-patterning inspection of wafers relies on the fact thatlight is scattered mainly from anomalies present on the generally flatand smooth surface of the non-patterned wafer. Thus, any detection ofscattered light may be indicative of a defect.

[0005] However, when inspecting patterned wafers, scattered light can becaused by the pattern. Therefore, the detection of scattered light isnot necessarily indicative of a defect. In order to detect defects on apatterned surface, templates formed by signals representative ofdetected light components scattered from periodic features of thepattern (i.e. dies of the wafer) are typically constructed and compared.Differences between the signals are indicative of light scattered fromanomalies present on the surface of the article, and are thereforedetected as defects. Another technique, so-called “die-to-die”inspection, consists of comparing light scattered from an individual dieto that of its “neighbor”. Any detected difference in light componentsscattered from these two dies, is indicative of the absence or additionof some features in one of the dies as compared to the other, and istherefore considered to be a defect.

[0006] To facilitate meaningful signal comparison, i.e. to successfullyanalyze the signals associated with different dies and cells of thewafer, it is desirable for the light collection system to collect lightat one constant collection angle. The advantages of a single constantcollection angle based technique are disclosed, for example, in U.S.Pat. Nos. 4,898,471 and 5,604,585, both of which relate to systems fordetecting particles on the surface of a patterned article.

SUMMARY OF THE INVENTION

[0007] There is a need in the art to improve the conventional inspectiontechniques by providing a novel method and apparatus for opticalinspection of articles utilizing a variable angle design.

[0008] It is a major feature of the present invention to provide such anapparatus that enables the signal-to-noise ratio of the detected signalto be significantly increased.

[0009] The main idea of the present invention is based on the following.An article (e.g., wafer) under inspection is scanned region-by-region,and light scattered from each of the scan regions is collected with acertain maximum collection angle constant for each scan region. Themaximum collection angle is a solid angle having a certain value and acertain central direction defined by a light collecting optics. A filteris selectively operable in the optical path of the collected light forselectively separating therefrom at least one light componentpropagating with a predetermined solid angle segment of the maximumcollection angle.

[0010] If any unexpected difference (defect) is detected at a specificlocation or, on the contrary, a probability of a defect exists at aspecific location on the article (according to previous knowledge of thepattern structure of the article, which is typically the case), thislocation is inspected by varying (reducing) the collection angle. Atthis specific location, namely for this specific scan region, one ormore light components of the collected light could be captured anddirected on a detector, thereby increasing the signal-to-noise ratio ofthe detected signal. If the article has a repetitive pattern, thispattern scatters light into a specific angular range according to thespecific pattern characteristics. The signal-to-noise ratio of thedetected signal can be improved by preventing light propagating withinthis specific angular range from being detected.

[0011] There is thus provided according to one aspect of the presentinvention an apparatus for optical inspection of an article, comprisingan illumination unit generating an incident radiation and illuminating apredetermined region on the article, and at least one detection unit,wherein said at least one detection unit comprises:

[0012] (a) a light collecting optics that collects light scattered fromthe illuminated region with a predetermined constant maximum collectionangle;

[0013] (b) a filter selectively operable in the optical path of thecollected light for selectively separating therefrom at least one lightcomponent propagating with a predetermined solid angle segment of themaximum collection angle; and

[0014] (c) detector having a sensing surface for receiving collectedlight and generating data representative thereof.

[0015] The illumination unit may be oriented so as to illuminate thearticle either normally or at a grazing angle. The at least onedetection unit operates in a perspective dark field imaging mode.

[0016] The filter may comprise a mask assembly. The mask assembly may becomposed of a plurality of different masks mounted so as to enable aselected one of the masks to be installed in the optical path of thecollected light propagating towards the detector. The filter may be aprogrammable LCD or a micro electro-mechanical structure (MEMS).

[0017] The light collecting optics is designed so as to form an angularimage of the illuminated region. The term “angular image” signifies suchan image of an object, wherein each point of the image corresponds to anangle formed by light coming from any point of the object. In otherwords, the collected light ensuing from the light collecting optics isrepresentative of the angular image of the illuminated region formed bythe light scattered from the illuminated region and propagated with thecertain constant solid angle. The collected light representative of theangular image of the illuminated region is transmitted to the detector.

[0018] The light collecting optics comprises first and second optics.The first optics defines the maximum collection angle and is capable offorming a real image of the illuminated region, while the second opticsis capable of forming from this real image an angular image of theilluminated region. To ensure that the real image is formed from lightcomponents coming from the illuminated region only, the light collectingoptics also comprises a slit with a shape and dimensions substantiallyidentical to those of the real image which is mounted substantially atthe expected location of this image. This enables any light componentcoming from a location outside the illuminated region to be preventedfrom reaching the detector. Hence, the signal-to-noise ratio of detectedlight is increased even more, considering “noise” as any light componentother than that scattered from the illuminated region with the maximumcollection angle.

[0019] Preferably, the detection unit also comprises an additionaloptical system accommodated in the optical path of the collected lightpropagating towards the detector, and is capable of directing said lightonto the entire sensing surface of the detector. This enables anundesirable dependence between the detected differences in outputsignals produced by the detector and typical non-uniformity of itssensitivity distribution to be avoided. Preferably, this additionaloptical system utilizes a telecentric-imaging mode.

[0020] Thus, according to another aspect of the present invention, thereis provided an apparatus for optical inspection of an article,comprising an illumination unit generating an incident radiation andIlluminating a predetermined region on the article, and at least onedetection unit, wherein said at least one detection comprises:

[0021] a light collecting optics that collects light scattered from theilluminated region with a predetermined constant maximum collectionangle;

[0022] a filter selectively operable in the optical path of collectedlight for selectively separating therefrom at least one light componentpropagating with a predetermined solid angle segment of the maximumcollection angle; and

[0023] an optical system imaging collected light onto the entire sensingsurface of a detector.

[0024] The detection unit may also comprise a polarizer accommodated soas to be displaceable between its two extreme positions, being,respectively, in and out of the optical path of the collected lightpropagating towards the detector. The polarizer, when positioned in theoptical path, may also be displaceable so as to change the orientationof the plane of its preferred transmission.

[0025] Preferably, the detection unit also comprises an imagetransmission means capable of transmitting the collected light to adesired location of the detector in a manner to avoid anydistance-introduced changes of the image. The image transmission meansmay comprise an imaging fiber bundle formed of a plurality of coherentfibers, or alternatively, may comprise relay optics.

[0026] The present invention, due to the above light collectingtechnique, allows for significantly increasing signal-to-noise ratio inthe detected light, “noise” being considered as any light componentother than those scattered from the illuminated region with the maximumcollection angle. By using more than one detection unit constructed asdescribed above and having light directing optics positioned so as tocollect light propagating with different azimuth angles, the defects onthe surface of the article are not only detected, but are alsosuccessfully classified. The term “azimuth angle” signifies an anglebetween the direction of an incident beam and that of a returned beam,as seen from the top view. Additionally, due to the provision of thefilter, the present invention allows for varying the azimuth and/orelevation angles from their maximum values defined by the lightcollecting optics. The term “elevation angle” signifies an angle betweenthe direction of a scattered light component and the scattering surface,as seen from the side view. This angles' variation enables differencesin the light components representative of the certain angular image ofthe illuminated region, but propagating with different solid anglesegments of the maximum collection angle, to be detected. The provisionof the polarizer enables the signal-to-noise ratio to be increased evenmore, considering the “polarized noise”.

[0027] More specifically, the present invention is used for inspectingwafers and is therefore described below with respect to thisapplication.

BRIEF DESCRIPTION OF THE DRAWINGS:

[0028] In order to understand the invention and to show how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0029]FIG. 1 is a schematic block diagram illustrating the maincomponents of an apparatus constructed according to one embodiment ofthe invention;

[0030]FIG. 2 is a schematic block diagram illustrating the maincomponents of an apparatus constructed according to another embodimentof the invention;

[0031]FIG. 3 is a schematic block diagram more specifically illustratingthe main components of a detection unit, constructed according to oneexample of the present invention, suitable for the apparatus of eitherof FIGS. 1 or 2;

[0032]FIG. 4 is a beam diagram illustrating the main principles ofoperation of light collecting optics of the detection unit of FIG. 3;

[0033]FIG. 5 more specifically illustrates a mask assembly suitable forthe detection unit of FIG. 3;

[0034]FIGS. 6 and 7 are schematic block diagrams illustrating the maincomponents of detection units constructed according to two more examplesof the present invention, respectively, suitable for the apparatus ofeither of FIGS. 1 or 2;

[0035]FIG. 8 is a beam diagram illustrating the main principles ofoperation of imaging optics of the detection unit of FIG. 3; and

[0036]FIG. 9 is a schematic block diagram of the main components of adetection unit according to yet another example of the presentinvention, suitable for the apparatus of either of FIGS. 1 or 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0037] Referring to FIG. 1, there is illustrated an optical inspectionapparatus, generally designated 1, associated with a wafer 2 to beinspected. The upper surface 2 a of the wafer 2 may or may not be formedwith a pattern, which is therefore not specifically shown. The apparatus1 comprises an illumination unit, generally at 4, and four detectionunits, generally at 6, operating in a perspective dark field imagingmode. Output circuits (not shown) of the detection units 6 are coupledto a processor 7. which is appropriately equipped with hardware and isoperated by suitable software, so as to successfully analyze data comingfrom the detection units 6. The construction and operation of theprocessor 7 do not form a part of the present invention and may be ofany known kind.

[0038] The illumination unit 4 comprises a light source 8, for example alaser, emitting a beam of light, generally at 10, and a light directingoptics 12. The light directing optics 12 comprises a suitable scanningmeans 14 (for example an acousto-optic element) mounted in the opticalpath of the beam 10, and a focusing optics 15. The acousto-optic element14 causes the beam 10 to scan along the wafer. Obviously any othersuitable scanning means may be used, for example a rotating mirror. Itshould be noted that the provision of any scanning means is optional,and a non-scanning beam could be used for the same purpose, i.e. forilluminating a line on the surface of the wafer. The focusing optics 15is typically a lens or a plurality of lenses (not shown). The scanningmeans 14 and focusing optics 15 operate together to focus the beam 10onto a scan line S (constituting an illuminated region) on the surface 2a. The propagation of light is shown here schematically solely in orderto facilitate the illustration of the main components of the apparatus1.

[0039] The construction and operation of the illumination unit 4 areknown per se and therefore need not be described in more detail, exceptto note the following. According to the present example of FIG. 1, theillumination unit 4 provides the scanning of the article 2 at asubstantially grazing angle relative to its surface 2 a. It isunderstood, although not specifically shown, that the light source 8 maybe accommodated at any other appropriate location relative to thesurface 2 a, provided the light directing optics 12 includes a mirror orthe like to form the desired angle of incidence of the scanning beam 10.

[0040]FIG. 2 illustrates an apparatus, generally designated 16, having asomewhat different construction in comparison to that of the apparatus1. The same reference numbers are used to indicate those components thatare identical in the devices 1 and 16, in order to facilitateunderstanding. As shown, an illumination unit 17 of the apparatus 16, asdistinct from that of the apparatus 1, provides a substantially normaldirection of the incident light 10 relative to the surface 2 a.

[0041] In both examples the laser beam 10 is caused to move in ascanning direction (i.e. along the X-axis), while the wafer 2 issupported on a translational stage (not shown) for movement along theY-axis. Each of the detection units 6 operates in a perspective darkfield imaging mode, i.e. collects light components, generally at 11,scattered from the surface 2 a at azimuth and elevation different fromthose where the most specular reflection occurs, wherein the specularreflected light propagates within a certain solid angle.

[0042] The four detection units 6 have similar constructions andtherefore the main components of only one of them are illustrated inFIG. 3. The detection unit 6 comprises a light collection system 18 anda detector 20 which may be of any suitable kind, for example aphotomultiplier tube (PMT) whose construction and operation are knownper se.

[0043] In the exemplified preferred embodiment, the light collectionsystem 18 includes collecting optics 22, a coherent (imaging) fiberbundle 26 (constituting image transmission means) and a mask assembly 28(constituting a filter) which is followed by an optical system 30. Whilenot a necessary element, the preferred embodiment includes a polarizer24 inserted in the light path of the optics 22.

[0044] The principles of operation of the coherent fiber are known perse and therefore need not be specifically described, except to note thefollowing. A coherent fiber bundle is used for transmission of images.The relative positions of individual fibers in the coherent fiber bundleis maintained, and, due to this fact, the fiber bundle provides desiredlinear correlation between input and output signals. The constructionand operation of the mask assembly 28 and optics 30 will be describedmore specifically further below with references to FIGS. 5 and 6,respectively.

[0045] The incident light 10 impinges onto the scan line S and isscattered by the pattern and anomalies that may occasionally occur onthe surface 2 a within the scan line S. The light components 11 that arescattered from the scan line S and propagate with a certain solid angleare collected by the optics 22 and directed into the polarizer 24. Thepolarizer 24 is operated by a suitable driver (not shown) for movementin a manner to be positioned either in or out of the optical path of thelight ensuing from the collecting optics 22. Additionally the polarizer24, when being installed in the optical path, is rotatable in a mannerto change the orientation of the plane of its preferred transmission(preferred polarization). The displacement of the polarizer 24 affectsthe light 11 a passing therethrough. Since the pattern may have highpolarization degree representing a so-called “polarized noise”, thedisplacement of the polarizer affects the signal-to-noise ratio in thelight component 11 a propagating towards the detector. Additionally, alearning mode may be applied to the specific kind of article prior toits inspection. This enables a certain polarization signature in thelight component returned from the pattern to be expected and, therefore,by changing the orientation of the polarizer 24, to detect differences(defects), if any.

[0046] Turning now to FIG. 4, the light collecting optics 22 comprises afirst lens assembly 32 (constituting a first optics) and a second lensassembly 34 (constituting a second optics), and a slit 36 located in afront focal plane P of the lens assembly 34. The light propagationthrough the lens assemblies 32 and 34 defines an optical axis OA. Thelens assembly 32 is positioned to collect light components scatteredfrom the scan line S and propagating with a certain solid angle. Thedimensions of a first glass surface 32 a of the lens assembly 32 definesthe value of the solid angle of collection of the optics 22, which isthe maximum collection angle of the entire detection unit 6.

[0047] The light collecting optics 22 is designed so as to form anangular image of the scan line S which is transmitted towards thedetector 20 through the fiber bundle 26. The lens assembly 32 isdesigned to form a real image S′ of the scan line S in an image planeIP. The image plane IP substantially coincides with the front focalplane P of the lens arrangement 34. The slit 36 is shaped anddimensioned similar to that of the real image S′, and is installed atthe location of the expected image. As for the lens assembly 34, itforms from the real image S′ of the scan line S an angular image S″thereof. Each point of the angular image S″ is formed by a lightcomponent coming from any point of the scan line S and propagating witha segment of the maximum collection angle. Each such segment of themaximum collection angle is transmitted towards the detector through oneor more corresponding fibers of the fiber bundle 26.

[0048] The provision of the slit 36 is based on the following. Theillumination unit, due to unavoidable reflection occurring therein,typically produces an additional undesirable image of the scan line (aso-called “ghost”) on the surface of the wafer. This undesirable imagewould also generate returned light propagating towards the lightcollecting optics. The slit 36 so designed and positioned allows thepassage of the collected light representative of the real image S′ ofthe scan line S to the lens assembly 34, and blocks the passage of anylight other than that associated with the image S′.

[0049] A surface 26 a defined by the front end (with respect to lightpropagation) of the fiber bundle 26, is located in a focal plane FP ofthe entire collecting optics 22, defining thereby the most desirablelocation for the system's entrance pupil. Such a design of thecollecting optics 22 and its position relative to the fiber bundle 26enable the resolution of the light collection system 18 to besignificantly increased, and allows for optimizing the fiber bundleoperation. The diameter of the fiber bundle 26 preferably slightlyexceeds that defined by the cone of the maximum collection angle, so asto allow all the collected light beams (i.e. all the solid anglesegments of the maximum collection angle) to enter the fiber bundle 26.

[0050] Thus, the light collecting optics 22 forms the angular image S″of the scan line S, and directs the collected light representative ofthis image into the fiber bundle 26. Each point of the image S″ isrepresentative of a certain angle of propagation of the collected lightcomponent (within the maximum collection angle) scattered from any pointon the scan line S. Each angular light component is projected onto acorresponding point (fiber) on the input surface 26 a of the fiberbundle 26, and is transmitted by the fiber bundle into the same angleensuing from the corresponding point on an output surface of the fiberbundle.

[0051] The above design of the light collecting optics 22 enables tosignificantly improve the signal-to-noise ratio of the collected lightentering the fiber bundle 26, “noise” being considered as any lightcomponent other than those scattered from the scan line within themaximum collection angle and forming the desired angular image S″ of thescan line S. Moreover, this technique ensures the same field of view ofall points along the scan line S, and of all the detection units 6,thereby increasing the signal-to-noise ratio in difference-indicativesignals that can be detected when comparing data generated by all thedetection units 6.

[0052] Turning now to FIG. 5, the mask assembly 28 is composed ofseveral differently designed masks, seven in the present example,designated respectively 28 a-28 g, which are formed in a commondisc-like opaque plate 40. This may be, for example, a metal disk withholes or a glass-with-chrome mask. The disc 40 is operated by its motor,which is not specifically shown here, for rotation in a planeperpendicular to the long axis of the fiber bundle so as to selectivelylocate the desired one of the masks in the optical path of thepropagating light 11 a (FIG. 3). Hence, in the operational position ofthe mask assembly 28, one of the masks 28 a-28 g is installed in theoptical path of the light ensuing from the fiber bundle 26.

[0053] Each of the masks 28 a and 28 e represents an aperture thatdefines a certain solid angle of collection smaller than the maximumcollection angle defined by the first glass surface 32 a of the lensarrangement 32. By placing one of the masks 28 a and 28 e in the opticalpath of the light ensuing from the fiber bundle 26, one certain solidangle segment of the maximum collection angle may be picked out.Generally speaking, each of the masks defines at least one transmittingregion R_(t) with respect to the collected light. Alternatively,although not specifically shown, a single mask having variabledimensions could be used.

[0054] The masks 28 b, 28 c, 28 d, 28 f and 28 g have differentpatterns, each formed of regions R_(t) and R_(b) which are,respectively, transmitting and blocking with respect to the lightimpinging on the mask. Each of these masks, when in its operativeposition (i.e. located in the optical path of the light ensuing from thefiber bundle), is capable of cutting off and picking out one or moresolid angle segments.

[0055] More specifically, the mask 28 c cuts off a central portion(defined by the blocking region R_(b) of this mask) of the solid angleformed by light impinging on the mask, while allowing the propagation ofa periphery cone segment (defined by the transmitting region R_(t) ofthe mask). The mask 28 b would pick out two solid angle segments definedby the dimensions of two transmitting regions R_(t), wherein these solidangle segments are spatially separated in accordance with the dimensionsof the blocking regions R_(b). The mask 28 d would pick out threespatially separated solid angle segments, allowing their propagationtowards the detector. The masks 28 f and 28 g would, respectively, pickout two and three spatially separated solid angle segments, but by meansof cutting off one and two elongated portions of the incident solidangle.

[0056] The provision of the mask assembly comprising several differentmasks allows for selectively varying the collection angle of the entiredetection unit 6, thereby enabling the differences, if any, to bedetected. To this end, knowledge of the wafer's pattern prior to itsinspection is desired, so as to apply the angle variation to thoselocations on the wafer where the existence of defects is possible, orwhere an “unexpected” defect-indicative signal is suddenly detected. Itis important to note that such a mask assembly provides for solid anglevariations simultaneously along two mutually perpendicular axes, shownas A₁ and A₂ in FIG. 5. By selecting an appropriate mask, the specularlyreflected light components, for example associated with 45° or 90°geometries of the pattern, could be prevented from being sensed by thedetector 20.

[0057] According to the above example of FIG. 3, the mask assembly 28 ismounted in the proximity of the output surface 26 b of the fiber bundle26. FIG. 6 illustrates another example of a detection unit, generallydesignated 106, suitable for use with the inspection apparatus 1 or 16.Similarly, those components which are identical in the units 6 and 106,are identified by the same reference numbers. In the detection unit 106,the fiber bundle 26 is located directly after the mask assembly 28. Inthis case, the mask assembly 28 is located in the focal plane FP of thelight collecting optics 22. When the mask assembly is mounted upstreamof the fiber bundle, the light component propagating within the selectedcollection angle is transmitted by the corresponding fiber towards thedetector. It should be noted that in the example of FIG. 6, the fiberbundle 26 may and may not be an imaging fiber.

[0058] Generally, the dimensions of a selected mask depend on thenumerical aperture of an objective lens used in the optical inspectionapparatus. Hence, the filter should preferably comprise a set of masks(which may or may not be made in a common disk) to meet the requirementsof all possible objective lenses.

[0059] It should be noted, although not specifically shown, that a maskassembly comprising a plurality of various masks may be designed like acontinuous pattern formed by transparent and opaque regions andextending along a circumferential region of a disc-like plate. Eachportion of the pattern is formed by a different combination of locallyadjacent transmitting and opaque regions, and represents one mask fromthe plurality of different masks formed by different portions of thepattern. One transmitting/opaque region, whilst being combined with itsleft-side neighboring region and whilst being combined with itsright-side neighboring region, may form two different portions of thepattern, respectively. By rotating the disk about its axis, a differentportion of the pattern (mask) is located in the optical path of thecollected light impinging on the disk. Mask 28 g in FIG. 5 depicts suchan arrangement.

[0060] It should also be noted that such a mechanical filter may bereplaced by a micro electromechanical structure (MEMS) or a programmableliquid crystal display (LCD) whose construction and operation are knownper se. The LCD segments are light valves which, when in the open mode,transmit or reflect light (depending on the kind of a liquid crystalmaterial used therein), and, when in the closed mode, block light. Aspatial filter in the form of a transmitting LCD is disclosed forexample in U.S. Pat. No. 5,276,498.

[0061]FIG. 7 illustrates by way of a block diagram another example of adetection unit, generally at 206. Here, a mask assembly 123 is either areflective LCD or MEMS, the fiber bundle 26 being therefore positionedso as to receive collected light reflected from the filter 128. In theexample of FIG. 7, similar to that of FIG. 6, the fiber bundle 26 may beof a non-imaging type.

[0062] Reference is now made to FIG. 8 illustrating the main operationalprinciples of the optical system 30. The optical system 30 projects thelight collected by the mask assembly 28 onto the entire sensing surface20 a of the detector 20. The optical system 30 comprises a suitablenumber of lenses, two lenses 42 and 44 in the present example, andpresents a telecentric imaging system, wherein the surface 20 a ispositioned at the location of the system's entrance pupil. Theprinciples of telecentric imaging optics are known per se and thereforeneed not be more specifically described, except to note that such opticsavoids distance-introduced magnification changes and maintains the samemagnification of the image over a wide range of distances along theoptical axis of the system. As known, light rays ensuing from each fiberF_(i) propagate within a certain cone C_(i) (solid angle). Chief rays CRof these light cones C_(i) pass through the center of the entrance pupil20 a. The light cone C_(i) ensuing from a point on the surface 26 b ofthe fiber bundle 26, reaches the detector unit 20 and illuminates theregion 20 a. Thus, the optical system 30 projects each such light coneC_(i) (i.e. images each point on the surface 26 b) onto the same regionof the sensing surface.

[0063] The provision of the optics 30 that projects the light collectedby the mask assembly 28 onto the entire sensing surface 20 a isassociated with the following. The sensing surface of any detectortypically has non-uniform sensitivity distribution. If the spatiallyseparated light cones C_(i) were projected onto correspondinglyseparated regions on the sensing surface, detected differences in theoutput signals produced by the detector might have been associated withthe sensitivity differences of these regions and not with thedifferences in the scattered light. To avoid such an undesirabledependence between the output signal and the selected collection angle,optics 30 provides the illumination of the entire sensing surface byeach one of the collected light components (selected angles).

[0064] As further seen in FIG. 8, the operational mask is designed suchthat two spatially separated light components C_(i) (angles) ensuingfrom two fibers F_(i), respectively, are picked out and transmitted ontothe entire sensing surface 20 a of the detector. Hence, due to theprovision of the mask, one can choose a specific fiber to be imaged ontothe detector, thereby choosing the collection angle.

[0065]FIG. 9 illustrates yet another example of a detection unit,generally at 306, in which the fiber bundle 26 is replaced by relayoptics 46 (constituting image transmission means). The relay optics 46is typically a multilens system that serves for maintaining a certainnumerical aperture of the light propagation. The multilens system 46,similar to that of the fiber bundle 26, presents a physical opening thatlimits the amount of light that can pass through the system andtransmits the image of the scan line S onto a desired location. Asillustrated in the drawing in a self-explanatory manner, the telecentricoptical system 30 operates as described above for projecting marginalrays ensuing from the multilens system 46 onto the entrance pupildefined by the surface 20 a.

[0066] It should be specifically noted that the mask assembly 28 may beinstalled either in front of or after the relay optics 46. If the relayoptics 46 is positioned in front of the mask assembly 28, the firstglass surface of the relay optics is located in an entrance pupil of thelight collecting optics 22.

[0067] As to the polarizer 24, it is understood that when using acoherent fiber bundle 26 as an image transmission means, the polarizer24 should be accommodated in front of the fiber bundle, because opticalfibers may depolarize the polarized light. However, when using the relayoptics 46 as an image transmission means, both locations of thepolarizer 24 relative to the relay optics 46 (i.e. in front of and afterthe relay optics) are suitable.

[0068] All the detection units operate in the above-described manner forcollecting with their light collecting optics light scattered from thesame scan line S at the same maximum elevation angle and at differentmaximum azimuth angles and forming the desired image of the scan line S.The detectors 20 generate data (electrical signals) representative ofthe detected light components. These data are received by the processor7 and analyzed in a conventional manner so as to detect differencesindicative of real defects existing within the scan line S.

[0069] In view of the above, the advantages of the present invention areself-evident. The provision of so designed light collecting optics 22results in the increased signal-to-noise ratio in the collected light,which varies with the addressed wafer's layer. This is due to the factthat only those light components which are scattered from the scan lineS with a preset maximum collection angle and which form an angular imageof the scan line, are collected and directed towards the detector. Theprovision of the coherent fiber 26 or relay optics 46 provides foreffective (i.e. free of distance-introduced changes) transmission of theimage, thereby facilitating the mounting of the detection unit 20 at adesired location relative to the inspected wafer 2. The provision of thefilter (e.g., the mask assembly 28) allows for significantly improvingthe inspection of articles. By selecting an appropriate mask, theazimuth and/or elevation of the collected light could be varied fromtheir maximum values. The use of several detection units 6 observing thesame region on the wafer 2, allows for meaningful comparison of datarepresentative of light components, which are scattered by the samefeatures on the wafer but propagate in different directions. It is alsounderstood that by providing more than one dark-field imaging detectionunit, oriented and operated as described above, the existing defectscould not only be detected, but also successfully classified.

[0070] Those skilled in the art will readily appreciate that variousmodifications and changes may be applied to the preferred embodiments ofthe invention as hereinbefore exemplified without departing from itsscope as defined in and by the appended claims.

1. An apparatus for optical inspection of an article, comprising anillumination unit generating an incident radiation and illuminating apredetermined region on the article, and at least one detection unit,wherein said at least one detection unit comprises: (a) a lightcollecting optics that collects light scattered from the illuminatedregion with a predetermined constant maximum collection angle; (b) afilter selectively operable in the optical path of the collected lightfor selectively separating therefrom at least one light componentpropagating with a predetermined solid angle segment of the maximumcollection angle; and (c) a detector having a sensing surface forreceiving collected light and generating data representative thereof. 2.The apparatus according to claim 1, wherein the light collecting opticsforms an angular image of the illuminated region, and transmits lightrepresentative of said image towards the detector.
 3. The apparatusaccording to claim 1, wherein the detection unit operates in aperspective dark-field imaging mode.
 4. The apparatus according to claim1, wherein said incident radiation normally impinges onto the article.5. The apparatus according to claim 1, wherein said incident radiationimpinges onto the article at an acute angle.
 6. The apparatus accordingto claim 2, wherein the light collecting optics comprises a first opticsforming a real image of the illuminated region to a second opticsforming from said real image an angular image of the illuminated region.7. The apparatus according to claim 6, wherein said light collectingoptics also comprises a slit that has shape and dimensions substantiallyidentical to said real image of the illuminated region, and is mountedsubstantially at the location of said real image formed by the firstoptics.
 8. The apparatus according to claim 1, wherein the filtercomprises a mask assembly.
 9. The apparatus according to claim 8,wherein said mask assembly has a plurality of different masks, aselective one of the masks being installed in the optical path of thecollected light, when in the operative position of the filter.
 10. Theapparatus according to claim 9, wherein each mask defines at least onetransmitting region with respect to the collected light, said at leastone transmitting region corresponding to said predetermined solid anglesegment of the maximum collection angle.
 11. The apparatus according toclaim 1, wherein said filter comprises a programmable LCD.
 12. Theapparatus according to claim 1, and also comprising an imagetransmission means for receiving collected light and transmitting it toa desired location of the detector.
 13. The apparatus according to claim12, wherein said image transmission means comprises a coherent fiberbundle.
 14. The apparatus according to claim 12, wherein said imagetransmission means comprises a relay optics.
 15. The apparatus accordingto claim 1, and also comprising a polarizer device selectivelyinsertable into the optical path of the collected light.
 16. Theapparatus according to claim 15, wherein the polarizer, when being inthe optical path of the collected light, is rotatable to change theorientation of a plane of its preferred transmission.
 17. The apparatusaccording to claim 1, and also comprising an optical system, whichimages each selected solid angle segment of the collected light onto theentire sensing surface.
 18. The apparatus according to claim 1,comprising at least one additional detection unit substantiallyidentical to said at least one detection unit, wherein the at least twodetection units collect light scattered from said illuminated region andpropagating at different directions relative to the direction ofpropagation of said incident radiation.
 19. An apparatus for opticalinspection of an article, comprising an illumination unit generating anincident scanning radiation and Illuminating a predetermined region onthe article, and at least one detection unit, wherein said at least onedetection comprises: a light collecting optics that collects lightscattered from the illuminated region and propagating within apredetermined constant maximum collection angle; a filter selectivelyoperable in the optical path of collected light for selectivelyseparating therefrom at least one light component propagating with apredetermined solid angle segment of the maximum collection angle; andan optical system imaging at least a component of the collected light,onto the entire sensing surface of a detector.
 20. The apparatusaccording to claim 19, wherein said collected light is representative ofan angular image of the illuminated region.
 21. The apparatus accordingto claim 19, wherein the light collecting optics comprises a firstoptics forming a real image of the illuminated region to a second opticsforming from said real image an angular image of the illuminated region.22. The apparatus according to claim 21, wherein said light collectingoptics also comprises a slit that has shape and dimensions substantiallyidentical to said real image of the illuminated region, and is mountedsubstantially at the location of said real image formed by the firstoptics.
 23. The apparatus according to claim 19, wherein the filtercomprises a mask assembly.
 24. The apparatus according to claim 23,wherein said mask assembly has a plurality of different masks, aselective one of the masks being installed in the optical path of thecollected light, when in the operative position of the filter.
 25. Theapparatus according to claim 24, wherein each mask defines at least onetransmitting region with respect to the collected light, said at leastone transmitting region corresponding to said predetermined solid anglesegment of the maximum collection angle.
 26. The apparatus according toclaim 1, wherein said filter comprises a programmable LCD.
 27. Theapparatus according to claim 19, and also comprising an imagetransmission means.
 28. The apparatus according to claim 27, whereinsaid image transmission means comprises a coherent fiber bundle.
 29. Theapparatus according to claim 27, wherein said image transmission meanscomprises a relay optics.
 30. The apparatus according to claim 19, andalso comprising a polarizer device selectively insertable into theoptical path of the collected light.
 31. The apparatus according toclaim 30, wherein the polarizer, when being in the optical path of thecollected light, is rotatable to change the orientation of a plane ofits preferred transmission.
 32. A method for optical inspection of anarticle, comprising the steps of: (i) illuminating successive regions ofthe article; (ii) collecting light scattered from the illuminated regionat a predetermined constant maximum collection angle; (iii) selectivelyseparating from collected light at least one light component propagatingat a predetermined solid angle segment of the maximum collection angle;(iv) directing at least a separated light component of the collectedlight onto a detector.
 33. The method according to claim 32, whereinsaid collecting comprises: forming an angular image of the illuminatedregion.
 34. The method according to claim 32, wherein said at least aseparated light component of the collected light is directed onto theentire sensing surface of the detector.
 35. The method according toclaim 32, and also comprising the step of: transmitting at least aselected light component of the collected light to a desired location ofthe detector.
 36. A method for optical inspection of an article,comprising the steps of: illuminating successive regions of the article;collecting light scattered from the illuminated region at apredetermined constant maximum collection angle; separating fromcollected light at least one light component propagating at apredetermined solid angle segment of the maximum collection angle;imaging only said at least one separated light component onto the entiresensing surface of a detector.